1. Technical Field
The present invention generally relates to protective coatings for optical window substrates and, more particularly, to a protective and anti-reflection coating for optical windows employed in airborne and ground based tactical and surveillance sensors.
2. Discussion
Optical windows employed in airborne and ground based tactical and surveillance sensors are often exposed to hostile environments where they collide with rain and sand. The degree to which the window can withstand the impact of the rain and sand directly influences the operational envelope available for the sensor. Additionally, failed windows must be replaced which results in sensor idle time and maintenance expense.
Conventional optical windows begin to sustain impact damage at aircraft speeds well below Mach 1 and fail prematurely due to high speed rain/sand particle impact. Prior art anti-reflection coatings provide minimal resistance against impact damage. The impact resistance, or knoop hardness, of these coatings does not allow the windows to operate safely at high rain/sand impact speeds without significantly degrading optical performance.
Furthermore, boron phosphide and gallium phosphide coatings which do provide good impact resistance have a relatively narrow band gap and therefore provide only marginal optical transmittance over a relatively narrow optical bandwidth. For example, boron phosphide has an optical absorption property which makes it suitable for operation primarily in the long wave infra-red (LWIR) range. Therefore, these coatings are not suitable with the newer class of broadband, common aperture optical sensors which employ broadband optical window materials such as multi-spectral zinc sulfide and operate in bandwidths from the television (TV) to mid-wave infra-red (MWIR) or from the TV to LWIR.
The drawbacks of conventional coatings results in the need to frequently refinish and/or replace optical windows which causes high window life cycle costs. Moreover, the operational envelope for sensor systems employing prior art coatings is restricted by their relatively poor optical performance at high rain/sand impact speeds.
Accordingly, it is desirable to provide a combined protective and anti-reflection coating having excellent broadband transmittance and yielding significant impact protection for the optical window from high speed rain/sand particles.